CA1067350A

CA1067350A - Weather covers for tankers

Info

Publication number: CA1067350A
Application number: CA289,172A
Authority: CA
Inventors: Rolf D. Glasfeld; Jan G. Morrill
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1976-10-28
Filing date: 1977-10-20
Publication date: 1979-12-04
Also published as: GB1542745A; FR2369155B1; DE2748492A1; DK151473C; NL7711812A; IT1092185B; JPS6121872B2; FI64095B; DK477277A; NO773662L; NO145827C; NO145827B; JPS5355882A; DK151473B; DE2748492C2; FR2369155A1; PT67161B; FI773188A; FI64095C; ES463622A1

Abstract

ABSTRACT
A liquid-transporting cargo ship has a hull which sup-ports a main deck and several cargo tanks which extend above the main deck. A generally hemispherical cover surrounds the above-deck portion of each tank. The cover is formed of at least three self-supporting metal plating subsections, each of which is con-nected along its bottom edge to the main deck and along its lateral edges to a meridonal expansion member which is generally tubular in cross section. For cargo tanks designed to carry cryogenic liquid, the cover is made gas-tight, and an inert atmosphere is maintained in surrounding relation to each cargo tank.

Description

~7351) This invention relates to cargo ships and more par-ticularly to weather cove~s for liquid tankers having large tanks which extend above the main deck.
Tankers have been used for a long time to transport oil, gasoline and other chemicals which are co~nonly stored at ambient temperature. It has recently been found practical, and commercially feasible, to transport cryogenic liquids in ship-board tanks. One particular use of such ~hips has been to trans-port liquified natural gas (LNG), which remains in liquid form at a temperature below about -162C. at atmospheric pressure.
One of~the several different types of ships that have `~ been developed for carrying LNG utilizes large spherical tanks, and U.S. Patents Nos. 3,841,269, issued October 15, 1974 and 3,908,574, issued September 3, 1975, are illustrative of this ., , : .
general type of ship. The construction of a ship of this type takes into consideration a number of factors not associated with ~a~ standard OLl tanker design, such as the change in dimensions which results ~rom thermal contraction and expansion that occurs ; between the time when the tanks are full and at their cryogenic temperature and the time when they are empty and at ambient tem-perature,~as for example, during inspection or perhaps during ;~
ballast voyage.
These large spherical tanks extend above the main or ~.. ;
weather deck of the ship, and it i~ important that they be pro-25 ~ tected from the oaean salt spray and the like. Moreover, such tanks are thermally insulated, requiring an appropriate thennal - barrier between the cryogenic temperature o~ the liquid within the tank and the ambient temperature of the surrounding oceanic ~ atmosphere. It is also usually desired to maintain an inert '.J ' i 30 atmosphere about the exterior of the LNG tanks.

~Various types of weather covers have been proposed and ~ ~used in such ships which incorporate spherical tanks. One such '`'' ~b~. , . . ~ ~P
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ship included larye, generally cylindrical weather cover sec- -tions whieh fit over the uppermost portion of each tank, with lower cylindrical sections disposed between tanks, and with the eylindrical seetions respectively interconnected by truncated eonical sections. Other types of weather covers:have been pro-posed which are generally hemispherican and whieh use heavy meridional support members having horizontally extending inter-eonneetions to create a supporting framework on top of whieh the cover plates are disposed.
It is an object of the present invention to provide an ~ - -improved weather cover for liquid-earrying tanks that extend above the main deck of a ship. A further objeet is to provide an improved weather eover for protection of spherical cryogenie tanks installed on ships. Another object of the invention is to provide 15 improved weather covers for spherieal eryogenie tanks, whieh ~
eovers are of free-standing eonstruetion so that they ean be built ;
on land and then lifted aboard ship and appropriately installed ~-above the tank in question. A still further objeet is to provide ~
:: :
~ a weather eover of simple design whieh can provide a gas-tight :! 20 jaeket above a cryogenic tank and which will effeetively with-stand the stresses of oeean voyages.
. :. .,, ~
~ In one partieular aspeet the invention provides a :, ,J~ liquid-transporting eargo ship comprising a hull, a main deek, at ~
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, least one eargo tank for holding liquid disposed in the hull and ~
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¦ 25 extending above the main deek, ancl a free-standing eover sur-i~ rounding the above-deek portion of the tank, said eover beiny '.
.,J generally a seetion of a spheroidal surfaee and including at ~ least three subsections formed of metal plating which ean main-l . -.
,~ tain their shape without support, each subseetion being curved two direetions and eonneeted along its bottom edge to the main deek, and also ineluding at least three expansion members eurved along their axes to have the same clegree of curvature as the i 2 ~ . , .

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lateral edges of said cover subsections, which are joined to the lateral edges of two ad~acent subsections and formed so as to resiliently de~lect when subjected to stresses and thereby pro-tect the shape of the subsections.
In another aspect, the invention provides a ~ree-standing cover for the above-deck portion of a cargo tank aboard a ship, which cover is a shell-like section o~ a spheroidal sur-face comprising at least three subsections formed of metal plat-ing which can maintain their shape without support, each of which subsections is a section of a spheroidal surface curved in two directions that is designed for connection along its bottom edge to the main deck of the ship, and at least three expansion mem- ;
bers, each of which expansion members is curved along its axis to have the same curvature as the lateral edges of the cover sub-1 15 section, is joined to the lateral edges of two adjacent cover i~ subsections and is formed so as to resiliently deflect when subjected to stresses and thereby protect the shape of the cover i~ subsections.
The aforementioned objects and features of the inven-~, 20 tion will be apparent ~rom the following detailed desaription of a preferred embodiment of the invention when read in con-junctLon with the accompanying drawings wherein: -~; FIGVRE 1 is a perspective view of a ship at sea which incorporates ~ive large spherical tanks each of which is pro- `
tec~ed by an individual weather cover incorporating various eatures o~ the invention;
FIGURE 2 is an enlarged vertical sectional view taken .~ . , .
generally along the line 2-2 o~ FIGURE l;
FIGURE 3 is an enlarged elevational view, with portions , . :
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" ~6~73so broken away, of one of the weather covers shown on the ship of FIGURE l; ..
~IGURæ 3a is an enlarged fragmentary section view of an alternate embodiment of the weather cover shown in FIG. 3;
FIGURE 4 iS a plan view of the weather cover shown in : FIGURE 3;
FIGURE 5 is an enlarged-fxagmentary sectional view taken along the line 5-5 of FIGURE 4; and - ~
FIGURES 6 and 7 are views similar to FI&URE 5 showing : ;
.. 10 alternative constructions.
Basically, the invention provides a free-standing -: .
weather ~over designed to co~er the~portion of a large ~pherical tank which protrudes abo~e the main deck of an LNG tanker The .. :
: : .
il~ustrated weather cover is a section of a hollow sphere which ~is divided i-n~o a plurality, i.a., at least three, of ~ubsections ~and which subsections are interconnected by means of meridional , .
expansion members. The individual cover subsections are free-standing entities,~usua1ly~made of wel~e:d pl;ates of appropriate thickness, and when interconnected with one;another areate the : .
~20:~`free-standing weather cover. The expansion members provide the .
resilience:to;absorb the deformations which result ~om the hog-ging~ and .saggi~g of an oaean~g~i~g ship and which would otherwise ..
have a destruative ef~eat upon a totally rigid, unstiffened . ~ :
; c~ver,~and they are al~o capable of aacommodating any changes in :..
$~:~dimension which ~ight result from thermal expansion and contrac-tion.
~ Depicted in FIGURE 1 is a ship 11 which contains five '~$;` ~ spheriaal tanks each of which iis covered by protective weather cover 13. Each tank 17 is made of metal, preferably aluminum, : .
- : 30~ and~the weather co~er 13 protects the upper portion of the tank ~3 ~ ~ : 17 which~extends above the main deck 15 of the ship.

:~ ~ As best seen in FIGURE 2, each of the tanks 17 is ~;, ' . :
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spherical and is supported via an integral metal skirt 19 which is connected to the tank at about its equator. ~l~hough the structural details of the connection between the skirt and the tank are not shown, they may be of the general type disclosed in U.S. Patent No. 2,901,592, issued to Rossheim on August 25, 1959. The lower part of the metal skirt 19 is suitably con-nected, as by welding, to an appropriate location in the hull 21 of the ship 11. Although the tank 17 is illustrated with this preferred method of support via a skir~ should be under-stood that various alternative support arrangements for a large spherical tank, which are shown in the art, may also be used.
If desired, a layer of insulation 23 may also be ap~
plied to the interior surface of the hull 21 below the spherical ~ank 17 so that, if any leakage should unexpectedly occur, the cryogenic liquid can be v~porized upon the insulation without endangering the hull. The spherical outer surface of the tank includes a covering of an appropria~ely thick layer of thermal insulation of a suitable ma~erial, such as foamed polyurethane ~the exterior of which is covered wlth a suitable elastomeric vapor barrier, for example, butyl rubber, so that the tank 17 retains the smooth spherical appearance shown in FIGo 2. Alter-natively, it may be possible to appropriately insulate the in-terior surfaae o~ the me~al tank wall.!
Each o~ the spherical tanks 17 is formed with a cupola or dome 25 at its top through which all of the piping connections are~routed and through which there is manual access into the tank 17, for inspection purposes, for example, downward via a stalrway~provided in a central casing (not shown). As can be seen in FIGURES 1 and 2, the cupola 25 extends above the weather cover 13 through an opening 27 provided therein. Accordingly, , ~ .
the weather cover 13 i~ provided with a central upper ring mem-ber or collar 29;to the interior o~ which is connected an annular , . . .
~4-t73so plate 30 wherein the opening 27 is formed that is sized so as to fit about khe exterior of the cupola 25 and the piping connections protruding therefrom. As earlier indicated, the weather cover 13 is desirably self-supporting so that it can be constructed 5 in the shipyard on land and then lifted by a crane for installa- .
tion about the insulated cryogenic tank, after the tank has been .
installed in the hull 21 of the ship 11. Once in place in the ship hull, a suitable flexible seal 31 is installed between the .~ ~:
. the annular plate 30 and an annular plate 32 of smaller dimen-. 10 sion which is affixed to the cupola 25, to thus seal the region ;.. .. :
:. between the weather cover 13 and the exterior surface of the ¢upola 25 and insure gas tightness at this location, at least : :
.. in those instances where it is desired to maintain an inert at~
mosphere in the region between the interior of the weather cover 13 and the exterior of the cryogeni~c tank I7.
The.illustrated weather cover 13 has substantially ~
the shape of a hollow hemisphere up to the central collar 29, .. ~ :
although it has a height slightly less than the radius of the :
, , : . ~
sphere because the equator of the spherical cryogenic tank lies well below (or example, about 13 feet below) the~level of the main deck 15. The shape of.the weather cover 13 is:~enerally referred to as spheroidal which should be understood to mean that it can be preci ely a seotion of a sphere but may also have the :
shape of some other surface of revolution or may only approximate

2~5 ~:either of these. Althouyh the shape of the weather cover 13 ,~ ~ preferably should match the surface of the upper portion of the cryogenic tank li, so as to maintain a relatively uniform spac-ing therebetween and keep the volume which must be filled with cj , .
inert gas desirably low, deviations fxom this relationship can ~ 30 be tolerated. Likewise, if for some reason the cryogenia tank .`; itself did not have a precisely spherical upper section but in~

stead had khe shape of some other ~urface of revolution, there '. ~ . ' -:

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~0~7350 might be advantage in the weather cover having a matching shape;
however, the weather cover 13 could also have the shape of a section of a sphere.
As best seen in FIGURE 4, the weather cover 13 is made up of a plurality of subsections 35 which together constitute the spherical surface section. At least three subsections 35 are used, and it is unlikely that more than eight subsections would be employed. In the illustrated preferred embodiment, four quad-rants are employed. Each of the four quadrants is a self-sup-porting subsection 35 which is curved in two directions and whichis suitably connected, as by welding, to the main deck 15 along its bottom edge 37 and to the upper ring 29 along its upper edge 39. The upper ring or collar 29 may be formed from steel plate of appropriate width and thickness, which is bént to form a cyl-indrical surface having a diame-t~er suffici~nt to accommodate ~he apertured plate 30 that provides clearance about the cupola 25~ -Each of the lateral edges 41 of each quadrant 35 is suitably joined, as by welding, to an expansion member 43 which ex ends from the main deck 15 to the central collar 29 and which accord-. ~ ~ 20 ingly~has the same degree of axial curvature as the lateral edge of the~quadrant 35.
s~e~s;t~seen ~in FIGURE 5, the cross sectional shape of ~the illustrated expansion member 43 is circular, and the thick-ness of the metal from which the expansion member 43 is formed is~generally less than the thickness of the plate from which the self-supporting quadrant 35 is ~ormed. As can thu~ best be seen in FIGURE 4, each expansion member 43 is welded, or other-; wise suitably joined, to th~e lateral edges of ~wo adjacent quad-rants 35. The expansion members 43 primarily function to xelieve deforma~ions which are created as a result of shipboard movement.
.
Because the outer surface o~ the weather cover 13 will always be at ambient temperature and because there will be insulation be ,s :
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tween it and the cryogenic li~uid, substantial stresses should not be created in the weather cover as a result of change in temperature as long as all systems are working as planned.
The hogging and sagging of a ship of this length, which occurs in the fore-and-aft direction in heavy seas, places signif-icant stresses upon structures, such as this, located above the main deck 15. It has been found that the employment of the four expansion members 43, which are aligned in meridional direction from the main de~k 15 to the upper collar 29, effectively compen~
sate for the tendency of the plate-like quadrants 35 to move in response to the hogging and sagging of the ship. The expansion members 43 thus relieve the potentially destructive ~tresses which would otherwise require such substantial reinforcement or such a greàter thickness of the plate that a significant weight 15 and fabrication penalty would xesult. - -The expansion members 43 which are used have a cxoss sectional shape such that their sidewall portions, to which the lateral edges of the subsections 35 are attached can resiliently deflect toward~and away from each other without causing any per-manen~t deformation thereto. Shown in dotted lines in FIGU~ 5 - is the deflection which the opposite sidewall portions of the : .
circular expansLon member 43 would undergo when subjected to compressive stresses. In those expansion members 43 where the stresses are in the opposite direction, the cross section would elongate instead of contracting in the ~irection shown. To ob-tain maximum e~ectiveness in this respect, it is believed that the weather cover 13 should be aligned aboard ship with two of , :
~ the expansion members 43 aligned athwa~tship, and with the other .
two expansion members 43 accordingly lying fore-and-a~t, as il-; 30~ lustrated in FIGURE 1. It is with this orienta~ion that it is believed the best accommodation of deflections is accomplished by the expansion members 43. I, for example, only three sub-. . .
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~ L0~73S~
sections 35 were employed, then one of the expansion members 43 should be aligned fore-and-aft and the other two expansion mem-bers respectively disposed at 180 intervals.
The expansion members 43 are preferably generally tu-bulax, and most prefarably have a cross section which is a com-plete tube. Examples of some alternative tubular constructions are shown in FIGURES 6 and 7. In FIGURE 6, an expansion member 43a is illustrated which has a cross section in the form of an ., .
- elipse, the longer axis o which lies in the vertical plane.
FIGURE 7 shows a rectangular cross section tube 43b wherein again the longer dimension of the rectangle is in a vertical plane, and this construc~ion has certain advantages over a tube having a cross section which is~that of a circle or some other figure o~ revolution. An e~pansion member 43 having a cross section which is less than a completé tube may also be used; for example, a channel which resembles the FIGURE 7 expansion member - 43b with the bottom wall om~tted or a member~having the cross section of an inverted U ma~ be used~ Nontubular shapes which will have the re~uisite deflection characteristics may also be used;-~or example, the expansion member 43 could ba Z-shaped or S-shaPed in cr~ss section.
However, the inward or outward déflection mo~ement of the sidewall portions of such an open-tubular or nontubular ex-pansion member 43 would result in ~ome tendency towards rotation 2~5 at the point of welding between the respective lateral edge of the~plateliké cover subsection 35 and the sidewall portion of ; the expansion membex that might require additional strength at the lines of joinder. The dotted lines in FIGURE 7 illustrate both the inward and outward defleation of the rectangular cross section tube 43b and show that the movement of the subsections ;~; ~ ..
35 is directly inward or outward. Thus, it can be seen that one ... .

advantage of the;use of a closed or complete tube is to elimin-.. : .

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35~) ate any such rotative stress at the joint.
The expansion members 43 and the cover subsections 35 can be formed from any suitable metallic material which will have adequate strength and acceptable weight characteristics. ~ -Usually, but not necessarily, the cover subsections 35 and the expansion members 43 are made from the same material, and gen-erally steel is used. Because of their tubular shape, the ex-pansion members 43 are "softer" and deflect by bending to change shape while the cover subsections 35 maintain their spheroidal configuration. The expansion members 43 are made of a material having an appropriate wall thickness so that for a reasonable tube depth (or diameter), a given displacement does not induce excessive stresses in the expansion members. Usually the thick-~ ness will be less than that of the cover subsections. One ex-ample of a weather cover ~or a spherical tank having a diametermeasuring about 120 feet utilizes 9/16 inch steel plate to form the cover subsections whiI~ the expansion members 43 are steel tubes of circular cross section having a diameter of about 36 inches and a wall thickness of abou~ 3/8 inch. The cryogenic tank 17 which is protected by the weather cover is preerably formed from plates of aluminum or low-temperature, high-nickel content steel. When steel is employed for the wea~her cover 13, it is coated with a protective coa~ing that will render it res~istant to the corrosive effects of the salt water atmosphere 25~ to which it~will be constantly exposed. When iner~ing is de sired, a supply of inert gas or an inert gas generating plant 49 is provided on the ship to provide inert gas to blanket the spherical tanks in the regions below the weather covers and ad-'~1 ~ ` ' jacent the tanks below deck. ~;

To facilitate movement of ship's personnel between the .j,: : . , '~ tops or`cupolas ~5 of adjacent tanks 17, without the necessity ~ for ha~ing to descend to the deck and then climb the next tank _g_ .:
..

~0~'7350 a catwalk or walkway structure 45 is provided which bridyes the gaps between the four pairs of adjacent tanks. To accommodate the catwalk structure, the expansion members 43 are provided with flattened sections 47 (see FIG. 3) just adjacent the locations where they join the collar 29. The flattened sections 47 do not significantly affect the overall deflection characteristics of the expansion members 43 and allow the stable installation of the catwalk system 45. ~n addition, because space aboard any ship is at a premium, the tanks 17 will accordingly be located close together, and flattened vertical sections 48 may be provided in ; the forward and the aft e~pansion members 43 adjacent where the cover meets ~he main deck. The fla~tened sections 48 provide ` clearance to create a passageway athwart ships between adjacent weather covers 13 along the main deck 15.
. .
15Shown in FIGURE 3a is an alternative configuration of a weather cover 50, which is generally simil~ar to the shell con-s;truction shown in FIGURE 3j but which incorpora~es a urther `~ ~ stress-relieving member. The weather cover 50 incorpoxates a large diameter base or bottom ring 51 which is formed from a tube 20~ of circular cross section and which extends for 360 about the lower periphery-of the weather cover~ Accordingly, *he bottom edge of-each of the cover quadrants or subsections 53 is suitably joined, as by welding, to a location on the upper suxface portion of the base ring 51, and the bo~tom end of each of the expansion ~members 55 is suitably cut to mate with the annular surface o~
` Z
the~base tube to which it is suitably joined, as by welding. ~n ;the case of the weather cover 50, instead of attaching the bottom edges of the cover subsections directly to the main deck of th~
ship, the connection to the main deck of the ship is made indir-ectly via the base ring 51. The base ring provides the additional .~ :
expansion and contraction characteristics, similar to those pro~

~ided by the meridional members, and thus pr~vides additional pro-~ ' ', .
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~l~673SO ~ -tection against the creation of potentially destructive stress in the self~supporting plate-like cover subsections 53 as a re-sult of the hogging and sagging of a ship in heavy seas.
: Although the invention has been illustrated and des-S cribed with respect to certain preferred embodiments~ it should be understood that various changes and modifications as would be - .
obvious to one having the ordinary skill in this art may be made without deviating from the scope of the invention, which is de- :-fined solely by the claims appended hereto. Various of the fea-tures of the invention are set forth in the claims which follow.

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Claims

The embodiments in which an exclusive property or privilege is claimed are defined as follows.

1. A liquid-transporting cargo ship comprising a hull, a main deck connected to said hull, at least one cargo tank for holding liquid disposed in said hull and extending above said main deck, and a free-standing cover surrounding the above-deck portion of said tank, said cover being generally a section of a spheroidal surface and including at least three subsections formed of metal plating which can maintain their shape without support, each of which subsections is curved in two directions and is connected along its bottom edge to said main deck, and also including at least three expansion members each expansion member being curved along its axis to have the same degree of curvature as the lateral edges of said cover sub-sections, being joined to the lateral edges of two adjacent sub-sections and being formed so as to resiliently deflect when sub-jected to stresses and thereby protect the shape of said cover subsections.

2. A ship in accordance with Claim 1 wherein said expansion members are generally tubular in cross section and have a thickness less than the thickness of said cover subsec-tions.

3. A ship in accordance with Claim 1 wherein said expansion members have a circular or eliptical cross section and said cover subsections are joined thereto along diametrically opposite locations.

4. A ship in accordance with either Claim 1 or 2 wherein said expansion members have a rectangular cross section and said cover subsections are joined to opposite sidewalls along generally central locations therein.

5. A ship in accordance with any one of Claims 1 to 3 wherein a cupola is provided at the top of said cargo tank and wherein a circular opening is provided at the very top of said cover which opening is interior of a collar that is joined to the top of each of said expansion members and along the upper edge of each cover subsection.

6. A ship in accordance with any one of Claims 1 to 3 wherein said bottom edge of each of said cover subsections is directly connected to a base tubular ring and said base ring is connected to said main deck.

7. A ship in accordance with any one of Claims 1 to 3 wherein said cargo tank is adapted to carry a cryogenic liquid, wherein the joinder of said cover subsections and said expansion members is such as to render said cover gas-tight and wherein an inert atmosphere is maintained in surrounding relation to said cargo tank.

8. A ship in accordance with any one of Claims 1 to 3 wherein four cover subsections of equal size and four expan-sion members are provided and two of said expansion members are aligned fore-and-aft.

9. A free-standing cover for the above-deck portion of a cargo tank aboard a ship, said cover being generally a shell-like section of a spheroidal surface and comprising at least three subsections formed of metal plating which can maintain their shape without support, each of which subsections is a section of a spheroidal surface curved in two directions that is designed for connection along its bottom edge to the main deck of the ship, and at least three expansion members, each of which expansion members is curved along its axis to have the same curvature as the lateral edges of said cover subsection, is joined to the lateral edges of two adjacent cover subsections and is formed so as to resiliently deflect when subjected to stresses and thereby protect the shape of said cover subsections.

10. A cover in accordance with Claim 9 wherein said expansion members are generally tubular in cross section.

11. A cover in accordance with either Claim 9 wherein said expansion members are made of material having a thickness less than the thickness of said cover subsections.

12. A cover in accordance with any one of Claims 9 to 11 wherein said expansion members have a rectangular cross section, wherein said cover subsections are joined to opposite sidewalls along generally central locations therein in a manner to provide a gas-tight cover.

13. A cover in accordance with any one of Claims 9 to 11 wherein said expansion members are formed of closed tubes and wherein a base ring member of tubular construction is joined to the lower edge of each of said cover subsections.